Throughout this description and in the following claims, the gearshift referred to can be the rear gearshift (which is the one shown as a non-limiting example in the attached figures), which moves the chain among the different sprockets of the sprocket assembly associated with the rear wheel of the bicycle, or the front gearshift, which moves the chain among the different crowns of the crankset associated with the crank arms. The movement of the chain is typically actuated through the movement of a derailleur associated with an actuation kinematic mechanism.
For some time, bicycle gearshifts with motorized actuation have been commercialized, wherein the movement of the derailleur takes place by means of a drive member that is properly controlled, typically electrically.
In the development of gearshifts, a great deal of attention has been paid to the quality of the actuation operation, that is, the capability of the gearshift to carry out the gearshifting quickly and precisely.
However, in normal use, the performance of a gearshift can be compromised by knocks undergone by the gearshift itself, for example due to the bicycle falling down or even just to loading, unloading and transportation maneuvers on a vehicle. A particularly hard knock can in the most serious cases cause some members of the gearshift to break; less severe knocks, on the other hand, can cause small deformations or even only small movements of the members of the gearshift, often not even able to be detected immediately by the eye, but nevertheless such as to compromise the precision and therefore the correct operation of the gearshift. The same deformations or mutual movements can in other cases be caused during gearshifting by temporary blocking of the kinematic mechanism with respect to the drive member, as sometimes happens because of the mechanical interference between the chain and the sprockets (particularly when passing from one sprocket to another sprocket having a greater diameter).
Therefore, actuator devices have been developed which are provided with protection systems of the gearshift against knocks or undesired temporary deformations/displacements. Such knocks or undesired temporary deformations/displacements are indicated hereinafter, in brief, with the expression “excess forces”.
Prior art document EP 1970299 describes an actuator device for a motorized gearshift wherein, in order to provide protection against possible excess forces, a clutch device is provided between the actuation kinematic mechanism and the motor, a clutch device consisting of a driving member, a driven member and a torsion spring that pushes the driven member against the driving member. The driving member is fixedly connected to the motor, whereas the driven member is fixedly connected to the actuation kinematic mechanism and is defined substantially by a first-class lever pivoted at a pivot pin arranged parallel to the rotation axis of the motor. The torsion spring acts on an end portion of the lever, whereas an opposite end portion of the lever comprises a tooth that, in normal operating conditions, is housed in a seat formed in the driving member. In the presence of an excess force, a rotation force is generated on the driven member that, when such a force exceeds a predetermined threshold, causes a rotation of the driving member about the pivot pin, a relative rotation between driven member and driving member and the consequent exiting of the tooth from the seat. The excess force is thus absorbed by the system thus preventing it to be transferred to the members of the actuation kinematic mechanism.
The Applicant observed that the actuator device described in the aforementioned prior art document has a significant bulk, mainly due to the shape and arrangement of the lever that defines the driven member and to the arrangement of the driven member and of the torsion spring. Furthermore, the couplings between driving member and driven member and between torsion spring and driven member, being exposed to the external environment, can be a source of critical situations in the presence of dirt or water.
The present invention provides for an actuator device for a bicycle gearshift that is more compact and reliable than the one described above with reference to the prior art.